High density UO2 and high thermal conductivity UO2 composites by spark plasma sintering (SPS)

ABSTRACT

Embodiments of the invention are directed to a method for production of a nuclear fuel pellet by spark plasma sintering (SPS), wherein a fuel pellet with more than 80% TD or more than 90% TD is formed. The SPS can be performed with the imposition of a controlled uniaxial pressure applied at the maximum temperature of the processing to achieve a very high density, in excess of 95% TD, at temperatures of 850 to 1600° C. The formation of a fuel pellet can be carried out in one hour or less. In an embodiment of the invention, a nuclear fuel pellet comprises UO 2  and a highly thermally conductive material, such as SiC or diamond.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is a continuation application of U.S.Non-Provisional application Ser. No. 14/420,702, filed Feb. 10, 2015,which is the U.S. national stage application of International PatentApplication No. PCT/US2013/055132, filed Aug. 15, 2013, which claims thebenefit of U.S. Provisional Application Ser. No. 61/683,384, filed Aug.15, 2012, all of which are hereby incorporated by reference in theirentireties.

This invention was made with government support under DE-AC07-05ID14517awarded by the Department of Energy. The government has certain rightsin the invention.

BACKGROUND OF INVENTION

There are many types of nuclear fuels that can be fabricated for acommercial plant, including metals, metal oxides, and metal nitrides.Uranium dioxide (UO₂) is the most commonly used fuel material incommercial nuclear power reactors and is desired for its transientaccident resistance. UO₂ has the advantages of a high melting point,good high-temperature stability, good chemical compatibility withcladding and coolant, and resistance to radiation. The main disadvantageof UO₂ is its low thermal conductivity, for example, about 2.8 W/m-K at1000° C. During a reactor's operation, because of the low thermalconductivity of UO₂, there is a large temperature gradient in the UO₂fuel pellet, causing a very high centerline temperature, and introducingthermal stresses, which lead to extensive fuel pellet cracking. Cracksadd to the release of fission product gases after high burnup.Additionally, the high fuel operating temperature increases the rate offission gas release and fuel pellet swelling caused by fission gasesbubbles. The fission gas release and fuel swelling limit the life timeof UO₂ fuel in a reactor. In addition, the high centerline temperatureand large temperature gradient in the fuel pellets lead to a largeamount of stored heat and an increase of the Zircaloy claddingtemperature in a loss of coolant accident (LOCA). The rate ofZircaloy-water reaction, which generates potentially explosive hydrogengas, becomes significant at temperatures above 1200° C. The ZrO₂ layerthat is generated on the surface of the Zircaloy cladding affects theheat conduction and can cause a Zircaloy cladding rupture.

The thermal conductivity of UO₂ is affected by the changes that takeplace in the fuel upon irradiation while used. During irradiation,fission products accumulate in the UO₂ matrix, causing fuel swelling.Fission products dissolved in the UO₂ lattice serve as phonon scatteringcenters that reduce the thermal conductivity of the UO₂ fuel.Precipitated fission products have much higher thermal conductivitiesthan does UO₂ and provide a positive contribution to the thermalconductivity of UO₂ fuel. Fission product gases initially form inirradiated fuel as dispersed atoms within the UO₂ lattice, coalesce toform small bubbles that contribute to a reduction of the fuel's thermalconductivity by acting as phonon scattering centers. At temperaturesbelow 1000° C., uranium dioxide retains essentially all the fissiongases, but above this temperature, gases are released, and littlefission gas remains in those regions of the fuel at temperatures inexcess of 1800° C. Radiation damage from neutrons, α-decay and fissionproducts, increases the number of lattice defects, which contributes toa reduction of the thermal conductivity of UO₂ fuel. Theradiation-induced decrease in the thermal conductivity of UO₂ is largeat low temperatures. Oxygen defects are known to anneal at around 500 K,and uranium defects largely anneal at 1000 K, hence the majority ofchanges in the thermal conductivity of UO₂ are observed below 1000 K. Afuel with an increased thermal conductivity could allow the output of areactor to be increased while maintaining the desired fuel core andcladding temperatures.

Preparation of conventional UO₂ pellets consists of preparation of agreen body from powders and sintering the powder compact in a furnaceheated to and maintained at 1600-1700° C. for up to 24 hours in inert oroxidative environment. The initial UO_(2+x) powder is mixed with U₃O₈ toachieve hyper stoichiometry, generally with an optimal O to U ratio of2.25 which allows for enhanced sinterability of the UO₂ powder due tothe increased diffusivity of uranium atoms through vacancies. Additionof 3-5% U₃O₈ powder to UO₂ granules has also been found to be beneficialin reducing end-chipping and improving the pellet integrity aftersintering. The sintered pellets are reduced to stoichiometric UO_(2.00)by the procedure outlined in ASTM-C1430-07. Temperature ramp rates usingthe conventional method are limited to less than 5° C./minutes and itcan take up to seven hours to reach the desired sintering temperature of1700° C. and nearly as long for the furnace to cool to room temperature.The powder compact is not held in a confined container and, therefore,the dimensions of the sintered pellet cannot be maintained to tighttolerances from one sintering run to another. Sintered diameter of thepellet has also been found to be a linear function of green density. Toachieve desired dimensional tolerances, subsequent machining operations,such as grinding, are often employed. The long duration exposure to hightemperature during sintering allows formation of reaction products,which may degrade the properties and mechanical integrity of the pellet.

The present inventors have conducted research with the goal ofincreasing the thermal conductivity of UO₂ fuel pellets in a manner thathas little detrimental affect on the neutronic property of UO₂, asdisclosed in Tulenko et al., Nuclear Engineering Education ResearchProgram Project No. DE-FG07-04ID14598, Final Report, Oct. 14, 2007. Thedirection has been to incorporate a material with high thermalconductivity with the UO₂ pellet. A highly conductive material that hasbeen combined with UO₂ is silicon carbide (SiC), where a single crystalof SiC has a thermal conductivity that is 60 times greater than that ofUO₂ at room temperature and 30 times higher at 800° C. Silicon carbidealso provides a low thermal neutron absorption cross section, a highmelting point, good chemical stability, and good irradiation stability.The composite of SiC with UO₂ formed between SiC whiskers and UO₂particles or with SiC coated UO₂ has been examined. Coating was found tobe ineffective as the SiC precursor, allylhydridopolycarbosilane(AHPCS), oxidized by the UO₂ during coating and a CVD precursordecomposed without formation of SiC during that process. Ball millingwas required to form a homogeneous mixture from the SiC Whiskers andUO₂. Hot pressing at the relatively low temperature of 1200° C. to avoidreaction between the SiC and UO₂ and high pressure was required toachieve a dense pellet because the whiskers interfere with matrixparticle rearrangement during sintering. Scanning electron microscope(SEM) images of the sintered pellets showed grains did not form withsizes similar to grains of pure UO₂ pellets and that SiC whiskers areintact within the uranium oxide matrix. Thermal conductivity of thecomposite was not reported. Hence, there remains the goal of achieving agood quality nuclear fuel with improved stability and thermalconductivities.

BRIEF SUMMARY

An embodiment of the invention is directed to a method for preparing anuclear fuel pellet where a powder comprising a nuclear fuel is sinteredby spark plasma sintering (SPS) to a maximum temperature of 850 to 1600°C. where the rate of increase after achieving 600° C. is at least 50°C./minute with the maximum temperature held for 20 minutes or less. Acontrolled pressure of 25 to 100 MPa can be applied while holding themaximum pressure, to yield a nuclear fuel pellet with a density greaterthan 90% TD. The nuclear fuel comprises uranium oxide, uranium nitride,thorium oxide, plutonium oxide, and/or other fissionable isotope oxideor nitride. The powder can include a thermally conductive material witha thermal conductivity greater than 10 W/mK to result in the formationof nuclear fuel pellets, according to an embodiment of the invention,comprising UO₂ and a thermally conductive material having a thermalconductivity, where the fuel pellets have a density of at least 80% TDwith the thermally conductive material uniformly distributed through thefuel pellet. The thermally conductive material can be SiC, diamond, BeO,a metal, or a metal alloy.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows a schematic of a die assembly and sintering chamber of aspark plasma sintering (SPS) device, as used in embodiments of theinvention.

FIG. 2 shows an SPS parameter profile, where the set and measured(actual) temperature, die displacement, and pressure are tracked duringan exemplary ten minute sintering run using UO₂, according to anembodiment of the invention.

FIG. 3 shows a photograph of a fuel pellet consisting of UO₂ prepared bySPS, according to an embodiment of the invention.

FIG. 4 shows plots of displacement for various SPS densification runsfor fuel pellets consisting of UO₂ with different maximum temperaturesand heating rates, where runs indicate increased densification rates,and where runs including imposed controlled pressure are indicated byarrows indicating where the control pressure is applied, according to anembodiment of the invention.

FIG. 5 shows a plot of the percent of theoretical density (TD) for SPSdensification of fuel pellets consisting of UO₂ where runs havedifferent maximum temperatures, heating rates, and imposed controlledpressure, according to an embodiment of the invention, with runs withoutthe imposition of a controlled pressure indicated on the plot.

FIG. 6 shows plots of the densification rate versus temperature forvarious SPS densification runs with fuel pellets consisting of UO₂densified to different maximum temperatures and with different heatingrates, where plots show increased densification rate upon the impositionof a controlled pressure that is applied at the temperature indicated byarrows, according to an embodiment of the invention.

FIG. 7 shows plots of densification and densification rates versustemperature for SPS runs to produce fuel cells consisting of UO₂ withoutimposition of a controlled pressure, and with imposition of a controlledpressure, according to an embodiment of the invention.

FIGS. 8A and 8B show SEM micrographs of sintered SPS prepared fuelpellets consisting of UO₂ where the sintering maximum temperature is1150° C. with a controlled pressure held for five minutes achieving a96.3% TD, where FIG. 8A shows a fracture surface before thermal etchingand FIG. 8B shows the polished surface after thermal etching, accordingto an embodiment of the invention.

FIG. 9 shows SEM micrographs of surfaces of fuel cells consisting of UO₂prepared by SPS which show the grain grow for various isothermal holdtimes, as indicated on each micrograph with the resulting relativedensity and average grain size, according to an embodiment of theinvention.

FIG. 10 shows a plot of the average grain size versus hold time forthree different fuel pellets consisting of UO₂ having similar densitiesof about 97% TD, according to an embodiment of the invention.

FIG. 11 shows a plot of Vickers Hardness versus average grain size thatdisplays a Hall-Petch relationship for fuel pellets consisting of UO₂,according to an embodiment of the invention, using different loads of0.2, 0.5 and 1 kg while measuring the hardness.

FIG. 12 shows a plot of the Young's modulus for various densities forfuel pellets consisting of UO₂, according to an embodiment of theinvention.

FIG. 13 shows a plot density of sintered UO₂ pellets at differentmaximum sintering temperatures for various hold times, according to anembodiment of the invention.

FIG. 14 shows a plot of the average grain size of the sintered UO₂pellets for various hold times and maximum sintering temperatures,according to an embodiment of the invention.

FIG. 15 shows a plot for the evolution of average grain size versus thetheoretical density of the samples sintered using heating rate of 200°C./minute where at about a TD of 95% grain size increases onlymarginally, according to an embodiment of the invention.

FIG. 16 shows the effect of hold time and maximum sintering temperatureon the resulting O/U ratio of the sintered pellets, according to anembodiment of the invention.

FIG. 17 is a plot of the thermal diffusivity of pellets at 100° C., 500°C. and 900° C. for pellets processed according to an embodiment of theinvention for the maximum temperature, hold time and TD indicated.

FIG. 18 is a plot of the thermal conductivities of pellets at 100° C.,500° C. and 900° C. for pellets processed according to an embodiment ofthe invention for the maximum temperature, hold time and TD indicated.

FIG. 19 shows SEM image of the selected UO₂ pellets, according toembodiments of the invention where: Aa-Ac are the pellets in thedensification region with the similar grain size of around 0.4 m; Ba-Bcare from the grain growth region with the similar density of around96˜97% TD; and where Aa is a fracture surface and the remaining arepolished and thermally etched surfaces.

FIG. 20 is a SEM image revealing formation of intra-granular poresduring densification for a pellet of 77% TD, where the arrows indicatethe steps that result in the intra-granular pores.

FIG. 21 shows overlaid XRD peaks for starting powder, as-sintered pelletsurface and pellet after surface grinding to remove the surface layer,according to an embodiment of the invention.

FIG. 22 shows plots of thermal conductivity of the low-density (<90% TD)pellets at the temperatures of 100° C., 500° C. and 900° C. where asignificant increase in thermal conductivity with increasing densityoccurs and is most pronounced at low temperature.

FIG. 23 is a plot of thermal conductivities of high density (96%-98% TD)pellets versus the average grain size at the temperatures of 100° C.,500° C. and 900° C. where a dashed lines indicates the average value ofthermal conductivity of all samples prepared according to an embodimentof the invention and shaded areas indicate the breadth of literaturevalues for conventionally sintered UO₂ pellets.

FIG. 24 shows SEM images of SiC whiskers, top, and powder, bottom, usedfor the preparation of fuel pellets comprising UO₂ and SiC particles,according to an embodiment of the invention.

FIGS. 25A and 25 B show photographic images of UO₂—SiC composite fuelpellets formed by a) oxidative sintering and b) SPS, according to anembodiment of the invention.

FIG. 26 shows plots of the density of UO₂—SiC (10 vol %) composite fuelpellets sintered by SPS, according to an embodiment of the invention, oroxidative sintering at various temperatures.

FIG. 27 shows SEM images of polished surfaces of high density UO₂—SiC(10 vol %) composite fuel pellets from a) SiCw (whisker) and b) SiCp(powder) pellets showing uniform dispersion of SiC, according to anembodiment of the invention.

FIG. 28 shows SEM images of UO₂—SiC (10 vol %) composite fuel pelletsformed by: a) and c) oxidative sintering at 1500° C. for 4 hours withSiC whiskers and powder, respectively; and b) and d) SPS at 1500° C.with a five minute hold time with SiC whiskers and powder, respectively,according to an embodiment of the invention.

FIG. 29 shows an SEM image correlated with an EDS line scan across theinterface of UO₂—SiC grains in a composite pellet fabricated by SPS at1600° C., according to an embodiment of the invention.

FIG. 30 shows XRD spectra of UO₂—SiC (70 vol %) pellet sintered by SPS,top, and oxidative sintering, bottom, at 1600° C., where the peakscontained within the dotted circles indicate signals form a USi_(1.88)phase.

FIG. 31 shows bar graphs of UO₂ grain size in 10 vol % SiC compositefuel pellets where the SiC is added as a powder or whiskers and thegrain size for 100% UO₂ fuel pellets, for comparison, using oxidativesintering and SPS, according to an embodiment of the invention.

FIG. 32 shows plots of thermal conductivities of UO₂ and UO₂—SiCcomposite fuel pellets sintered by oxidative sintering or SPS, accordingto an embodiment of the invention.

FIG. 33 shows SEM images of UO₂-5 vol % SiC pellets with the SiCparticle mean diameter of (a) 0.6 μm, (b) 1 μm, (c) 9 μm, (d) 16.9 μm,and (e) 55 μm according to an embodiment of the invention, wheremicro-cracks originating from large size SiC particles are observed in(e).

FIG. 34 shows SEM images of UO₂-5 vol % SiC pellets with mean diameterof (a) 0.6 μm, (b) 1 μm, (c) 9 μm, (d) 16.9 μm, and (e) 55 μm, accordingto an embodiment of the invention, where micro-cracks in the matrix andbetween two SiC particles are identified by arrows.

FIG. 35 shows SEM images of UO₂-5 vol % SiC pellets, according to anembodiment of the invention, with interfacial dedonding for SiCparticles of size (a) 9 μm, (b) 16.9 μm, and (c) 55 μm.

FIG. 36 shows a plot of the thermal diffusivity of UO₂-5 vol % SiCcomposite pellets, according to an embodiment of the invention, withvarious SiC particle sizes as a function of temperature.

FIG. 37 shows a plot of the thermal conductivity of UO₂-5 vol % SiCpellets, according to an embodiment of the invention, with various sizesof SiC particles at the selected temperatures where dotted lines areliterature values of UO₂ thermal conductivity at each temperature.

FIG. 38 shows SEM images of UO₂—SiC composites, according to anembodiment of the invention, containing (a) 5, (b) 10, (c) 15, and (d)20 vol % of 1 μm SiC particles where circles are drawn aroundparticle-particle interaction sites.

FIG. 39 is a plot of the Relative density of UO₂—SiC composite pellets,according to an embodiment of the invention, containing variousfractions of 1 μm size SiC particles.

FIG. 40 shows plots of thermal diffusivity of UO₂—SiC composite pelletscontaining various volume fractions of 1 μm size SiC particles,according to an embodiment of the invention, as a function oftemperature.

FIG. 41 is plots showing the temperature dependence of specific heatcapacities of UO₂—SiC composite pellets, according to an embodiment ofthe invention, containing various volume fraction of 1 μm size SiCparticles.

FIG. 42 is plots of calculated and experimentally determined thermalconductivities of UO₂—SiC composites, according to an embodiment of theinvention, at selected temperatures with various volume fraction of 1 μmSiC particles, with UO₂ literature values at each temperature indicatedat Vf=0.

FIG. 43 shows SEM images of Interfacial contact between 1 μm SiCparticles in a UO₂-20 vol % SiC composite, according to an embodiment ofthe invention, where interfacial porosities are indicated by arrows.

DETAILED DISCLOSURE

Embodiments of the invention are directed to the preparation of superiorfuels for nuclear power plants comprising UO₂, other metal oxides, metalnitrides, or their composites with high thermal conductivity materials.The UO₂ can have up to 19.9 percent U-235 enrichment. The UO₂ cancontain other fissionable isotopes, such as thoria or plutonia. Thesuperior fuels are enabled by employing spark plasma sintering (SPS) asthe method of preparing the fuel pellets. Spark plasma sintering (SPS),also known as pulsed electric current sintering (PECS), is a fieldassisted sintering technique that allows production of fully densematerials while applying high heating rates and short dwell times. Apulsed DC current is passed through the punches, die, and, in somecases, the specimen, depending on its electrical properties. SPS is atechnique that has been developed for the rapid densification of ceramicmaterials, hard-metals, cermets, Al-based alloys, and other metallicpowders. In embodiments of the invention, fuel pellets are prepared atrelatively low temperatures in very short periods of sintering to yieldsuperior fuel pellet with large grain sizes and high hardness.Throughout the following description, the formation of structures andsintering phenomena are rationalized mechanistically, where themechanisms recited are consistent with the observations of theexperiments. However, embodiments of the invention are not limited bythe mechanism disclosed herein.

In an embodiment of the invention, the method of preparing UO₂comprising fuel pellets, is by SPS. In this method, a die assembly, asillustrated in FIG. 1, is loaded with a starting powder comprising UO₂,other metal oxide or metal nitride. The loaded die assembly is insertedinto a sintering chamber of a SPS and the chamber is depressurized, forexample, to less than 100 Pa, to less than 50 Pa, to less than 30 Pa, toless than 20 PA, to 10 Pa, to less than 10 Pa, or any pressure thatpermits sufficient removal of gas from within the loaded mixture. Apulsing current, for example, of 100 to 8000 A, of 200 to 2000 A, of 300to 1500 A, of 400 to 1000 A, of 600 to 800 A, or any current or currentrange that promotes densification of the loaded fuel pellet precursorcan be applied. An on-off current pulse can have any workable ratio, forexample, an on-off ratio of 12:2. An exemplary plot of temperatures andpressures applied in a typical SPS sintering run is shown in FIG. 2. Thedie assembly can be heated in a controlled manner with a heating rateof, for example, 50° C./min, 100° C./min, 150° C./minute, 200° C./min,or any other heating rate, where an optimal heating rate for a givenpressure is that by which one achieves an optimal pellet structure in aminimal period of time. Heating is maintained up to a desired maximumsintering temperature or to the vicinity of the maximum temperature, forexample, within about 100° C. or less of the maximum temperature, andheating at a lower second heating rate, for example one half or onequarter of the first heating rate to the maximum sintering temperatureof, for example, 850° C. to 1600° C., for example, 1050° C., 1035° C.,or 1045° C. The temperature is then held at the maximum temperature fora desired hold time, for example, 0.5 minutes to 20 minutes or more, forexample, 5 minutes. The pressure can be one that is inherently imposedby or on the die to maintain contact between the various surfaces of theassembly, and herein is recited as a ‘non-controlled’ pressure, forexample, a pressure of 14 MPa to 24 MPa during a portion of thesintering process, for example, all of the sintering time or, forexample, until a desired temperature is achieved. If desired, at a giventemperature during sintering, an additional uniaxial pressure, hereinrecited as a ‘controlled’ pressure, can be applied, for example, apressure of 20 to 80 MPa, for example, 40 MPa. By implementation of theSPS method, according to an embodiment of the invention, a fuel pelletwell suited for use as a nuclear energy generator can be prepared in avery short sintering time, for example, a period of less than two hours,including cooling to ambient temperature in about 40 minutes.

In embodiments of the invention, no alteration of the oxidestoichiometry by the mixing of powders is required to enhance thesinterability of the UO_(2+x), where a powder of any reasonablestoichiometry can be used. A traditional reduction of the oxide toUO_(2.00) is not required, according to an embodiment of the invention,where, by appropriate processing conditions for SPS, UO₂ powder can bereduced into the stoichiometric form of UO_(2.00) without any need for apost-sintering reduction in a H₂ atmosphere.

The fuel pellets, according to an embodiment of the invention, can be aUO₂ comprising fuel pellet, where the density is at least 80%theoretical density (TD) of the materials in their desired proportionsand at a desired temperature or range of temperatures; for example, afuel pellet consisting of UO₂ can achieve a density in excess of 95% TD,even up to 97% TD or more. The grain size of the UO₂ can be from 2 toabout 24 μm and even up to 100 m, and the porosity of the fuel pellet isprimarily from intra-granular pores and not from inter-granular pores.Fuel pellets with grain sizes of 6 to 9 m, which exhibit superiorthermal conductivities to pellets prepared by other methods, are readilyprepared in short production times. For a fuel pellet consisting of UO₂,according to an embodiment of the invention, inter-granular pores arenearly absent. Inter-granular pores, which decrease linearly with TD toabout 95% TD, are effectively absent above 96% TD. Inter-granular poresallow moisture absorption in fuel pellets and the degradation of thefunction of the fuel during operation of a nuclear power plant. Thepunch geometry of the SPS can be modified to permit sintered shapes notpossible by conventional sintering methods.

In an embodiment of the invention, the UO₂ comprising fuel pelletsfurther comprise a material with high thermal conductivity. The highlyconductive material has a thermal conductivity of at least 10 W/mK. Inone embodiment of the invention, the highly thermal conductivitymaterial is SiC. In another embodiment of the invention, the highlythermal conductivity material is carbon. The high thermal conductivitymaterial can be included in the UO₂ comprising fuel pellet at levels ofup to 30 vol % or more. In embodiments of the invention, the highthermally conductive material is homogeneously dispersed throughout thefuel pellet. Other highly thermally conductive materials that can beemployed according to an embodiment of the invention include BeO,metals, or metal alloys, for example, a metal included as microwires ornanowires. The metal can be uranium, uranium-metal alloy, for example,uranium silicide, another metal, or another alloy, for example, a metalor metal alloy with a melting point above 1100° C. The metal can be onethat does not oxidize readily on contact with UO₂ at elevatedtemperatures.

In an embodiment of the invention, UO₂ comprising fuel pellets can havean approximately three fold enhancement, or more, in thermalconductivity over traditional UO₂ fuel pellets. In an embodiment of theinvention, the fuel pellets comprise 90 to 99 vol % UO₂ and from 1 to 10vol % diamond particles ranging in size from 0.25 to 70 microns. Thediamond particles are evenly distributed in the UO₂ and the fuel pelletsdisplay greater than 90% TD. UO₂ particles and diamond particles can beblended, for example, by ball mill with the aid of blending agent, forexample, a volatile fluorocarbon liquid, prior to densification byoxidative sintering or by SPS.

Methods and Materials

Materials

Uranium dioxide powder was supplied by Los Alamos National Laboratory(Los Alamos, N. Mex.). The powder was reported to have a bulk density of2.3 g/cm³, tap density of 2.65 g/cm³, mean particle diameter of 2.4 μmand a BET surface area of 3.11 m²/g. The O/U ratio for the startingpowder was determined to be 2.11 by measuring the weight change beforeand after reducing the powder into stoichiometricUO₂ using ASTMequilibration method (C1430-07). In contrast to conventional oxidativesintering, where hyper-stoichiometric powder UO_(2.25) [U₃O₈+UO_(2.11)in 30:70 wt. ratio] is often used no UO_(2.25) was included and allsintering was performed using the as-received UO_(2.11) powders.

UO₂ Fuel Pellets

SPS Sintering was performed using a Dr. Sinter® SPS-1030 system, wherethe die assembly is illustrated in FIG. 1. As-received starting powderwas loaded into a graphite die of diameter 12.7 mm. The inner diesurface was covered by a thin (0.13 mm) roll of graphite foil to preventreaction of UO₂ with the die wall. Cylindrical graphite punches wereinserted into both ends of the die. The surface of each punch that comesin contact with the powder was coated with an aerosol of graphite (ZYPCoatings, Inc., Oak Ridge, Tenn.) for preventing undesirable reactionbetween the punch and the pellet as well as for easy release of the UO₂pellet from the die after sintering. The die was then wrapped with asheet of graphite felt to reduce heat loss from the graphite surface,decrease the thermal gradient between the surface of the die and thepellet, and protect the outer chamber from thermal radiation damage. Awindow was cut in the felt of approximately 10 mm×10 mm to allowtemperature measurement on the surface of the die by using a radiationpyrometer during the sintering process.

The die assembly was placed in the sintering chamber of the SPS and thechamber was depressurized to 10 Pa. A pulsing current of 600-800 A wassupplied at an on-off ratio of 12:2. The applied uniaxial pressure,temperature, z-axis motion of the punch, the chamber atmospherepressure, current, and voltage were continually recorded as a functionof time. A typical plot of some of the processing parameters during asintering run is provided in FIG. 2. The pyrometer lower limit ofdetection was 600° C. and the programmed heating schedule was initiatedonly after the temperature of the die surface, as measured through theopening cut in the felt, reached 600° C. Above this temperature, theheating rate was programmed at either 100° C./minutes or 200° C./minutesup to a temperature of about 50° C. below a desired maximum sinteringtemperature, and then at 50° C./minutes up to the maximum sinteringtemperature of 850° C. to 1525° C. The hold time at the maximumtemperature was varied between 0.5 minutes to 20 minutes for differentsintering runs. No programmed pressure, a ‘non-controlled’ pressure, of14 MPa to 24 MPa, was applied on the powder compact to maintain thecontact between various surfaces of the assembly until a specifiedtemperature was achieved, after which, an additional uniaxial pressure,a ‘controlled’ pressure, of 40 MPa was applied. Pressure and temperaturewere held constant for a desired time of 0.5 to 20 minutes, after whichthe pressure was released. During this procedure the Z-axis displacementof the punch indicates the densification of the UO₂ powder. Asignificant displacement occurs during the period when temperature risesabove 600° C. The displacement then remains constant until thecontrolled pressure is applied. Final densification occurs uponapplication of the peak controlled pressure of 40 MPa immediately beforeachievement of the maximum sintering temperature.

After release of the controlled pressure, the chamber was allowed tocool for one hour and the graphite die and sintered compact was removedfrom the SPS chamber. Pellets were polished using 240 grit sandpaper,which removed residual graphite foil and aerogel on the pellets'surface. Pellets were reduced to stoichiometric UO₂ following theprocedure described in ASTM C1430-07. Before micro-indentation hardnessmeasurements, pellet surfaces were ground and polished using 0.05 μmcolloidal silica with standard metallographic methods. Vickers hardnessmeasurements were conducted at 200 g, 500 g, and 1 Kg applied loads forperiods of 11 seconds, with at least five measurements at differentlocations on each pellet performed at each load. Ultrasonic measurementsin both longitudinal and shear wave velocity were conducted on eachpellet to determine the pellet's Young's modulus. Archimedes' method wasused to determine the density of each pellet. The pellet surfaces werepolished and thermally etched at 1400° C. for one hour to reveal grainboundaries. Scanning electron microscopy (SEM) was conducted to imagemicrostructural features, using an accelerating voltage of 15 KV andelectron beam current of 10-12 μA without a conductive coating on thesample's surface. Grain size was measured by the line-intercept methodusing ASTM E112 method covering over 100 grains in each sample.

The densification of the pellets was measured by the reduction in thelength of the green body pellet during sintering. FIG. 4 shows a fewselected plots of the die surface temperature and densification,ΔL/L_(o), of UO₂ compacts for various combinations of heating rate, holdtime, and maximum temperature during sintering; where ΔL is thedisplacement of the lower punch and L_(o) is the original green bodythickness before sintering. As revealed in FIG. 4, the densificationprocess depends on processing conditions. In general, an s-shape profilewas observed for densification in all exemplary runs with the exceptionof the run with a maximum temperature of 850° C. where no plateau isobserved. At furnace temperatures below 720° C., only limiteddensification was observed. As the temperature increased through adensification range of 720 to 1000° C., a rapid increase in density wasobserved. Increasing the temperature beyond this range did not result inan increase in density. Further densification occurs upon theapplication of a controlled pressure of 40 MPa, as is apparent from thesteep rise at the end of curves in FIG. 4. When the temperature wasincreased beyond 1350° C. a slight decrease in densification was noted,even with the controlled pressure applied, which is consistent withthermal expansion of the punch to a greater extent than is the shrinkageof the compact at these higher temperatures. Thus, for all the abovecases where the maximum sintering temperature was above 1000° C., thedensification behavior followed the s-shaped curve.

FIG. 5 plots final pellet density for various maximum sinteringtemperatures under various processing conditions. With a rapid heatingrate of 200° C./minutes and the minimal hold time of 0.5 minutes at amaximum temperature of 850° C. the density achieved for the fuel pelletwas 78.4% without and 86.5% with the imposition of a controlledpressure. By increasing the maximum sintering temperature andapplication of the controlled pressure greater fuel pellet densitieswere achieved, with 96.3% TD at 1050° C. with the controlled pressure.Beyond this temperature there was little variation in fuel pelletdensity, where densities of 97% TD to 97.6% TD were achieved at maximumsintering temperatures of 1500 to 1525° C. using a controlled pressureat temperatures above 1350° C. A fuel pellet sintered at 1525° C.without application of a controlled pressure showed a density of 97% TD.The greater effect of applying a controlled pressure at relatively lowtemperature, for example, 850° C., than at higher temperatures, forexample, above 1350° C., suggests that the controlled pressure is mosteffective while particles in the powders were still undergoing particleadhesion and rearrangement, considered the first stage of sintering. Inthe first stage, the controlled pressure results in more particlescontacting each other and neck formation between neighboring particlesis facilitated. Application of the controlled pressure only at hightemperature, for example, 1350° C., during the final stage of sinteringwhere densification is primarily diffusion dependent, is nearlyinsignificant at promoting further densification. This is consistentwith the data plotted in FIG. 6, where the densification rate as afunction of surface temperature is plotted, showing a rapid rateincrease above 700° C. and achievement of a rate maximum between 800 and900° C., where having a maximum sintering temperature of only 850° C.results in the densification dropping rapidly to zero upon achievementof the maximum temperature. For higher maximum sintering temperatures,the densification rate falls gradually to zero with little effect of themaximum sintering temperature above the ‘densification range’ displayedin FIG. 5. Increasing the heating rate to 200° C./minutes from 100°C./minutes results in like initial densification rate, but results in asignificantly larger maximum densification rate although the finaldensity of the pellets remained nearly the same for pellets heated tothe same maximum temperature. Application of controlled pressure nearthe end of the sintering phase results in an additional densificationrate increase, as can be seen in the curves of FIG. 6. No effect on thedensification rate was seen for different hold times.

FIG. 7 shows the effect of the controlled pressure, where densificationand densification rate are plotted against surface temperature for apellet sintered at a maximum temperature of 850° C., where UO₂ compactis the major densification phase with a high densification rate. Incontrast to densification where no controlled pressure is applied whereuniaxial pressure between 14-24 MPa occurs during the entire processingperiod, the controlled pressure is applied at 800° C., with a peakpressure of 40 MPa reached at 850° C. FIG. 7 reveals that without acontrolled pressure (dashed line), densification increases from around700° C. to the maximum sintering temperature and further increasesduring a 0.5 minutes hold time while the temperature drops to around825° C. The densification rate curve also reveals this phenomenon. Thesteep drop of the curve at 850° C. indicates that densification hasstopped. By applying a controlled pressure (solid line), higherdensification and a higher densification rate are achieved. A finaldensity of 86.5% is achieved by employing a controlled pressure whereasa density of only 78.4% is achieved where no controlled pressure isapplied.

Sinterability of UO₂ is attributed to an increase in the diffusion ofuranium ions at high sintering temperature under hold times of more thanthree hours using conventional or oxidative sintering methods. In thosecases, the onset of shrinkage is observed from 800-900° C. to yield amaximum density of 96.5% theoretical density (TD) when sintering up to1100° C. over more than three hours with an initial composition havingan O to U ratio of 2.22. Using a reducing atmosphere results in adensification onset between 1100 and 1150° C., where densification overa period greater than three hours and a maximum sintering temperature of1600° C. to attain 90% TD. In contrast, according to an embodiment ofthe invention, densification starts at a temperature of as little as720° C. with densification up to 96.3% TD at 1050° C. with a total runtime of 10 minutes with a hold time of 0.5 minutes at the maximumtemperature, as shown in FIG. 5. This result implies that diffusion ofuranium is not a major factor in densification because the entiresintering cycle was only 10 minutes and the period above 720° C. is onlyabout four minutes, which is consistent with the uniaxial pressure andthe pulsed current being the key factors favoring rapid densification.The pulsed current contributes to the densification with a microscopicdischarge occurring at the surfaces of the particles to promotedensification.

The microstructure of a pellet sintered at 1150° C. for five minutes isshown in FIGS. 8A and 8B, where FIG. 8A shows the fracture surface andFIG. 8B reveals the polished surface after thermal etching at 1400° C.for 30 minutes, where both micrographs reveal significant inter- andintra-granular pores. The density of the pellet was 96.3% TD and theaverage grain size was 2.9±0.3 m. Limited grain growth occurred duringthe SPS, primarily due to the rapid heating rate and short hold time.Because of rapid heating, coarsening during the lower temperature range,below 700° C., is bypassed rapidly and the densification occurs duringthe very short time at temperatures above 700° C. FIGS. 8A and 8B reveala large number of intra-granular pores. Unlike traditional sinteringprocesses where intra-granular pores are attributed to over sinteringwhen high temperatures and/or long sintering times are employed, whereinter-granular pores break from grain boundaries and migrate to theinterior of the grains rendering them difficult to eliminate due to alow lattice diffusion rate. This is not the case for fuel pellets,according to an embodiment of the invention, as the SPS process isunlikely to permit over-sintering at low temperatures in the shortsintering times. In the SPS process the high heating rate andapplication of the uniaxial pressure at the peak sintering temperaturemay promote neck formation along the inter-particle contacts and closedpores form from gas trapped between the particles. It is seen in FIG. 5that the maximum density was 97%, with the remaining 3% porosity beingprimarily intra-granular porosity.

Although the hold time has little effect on densification, grain growthduring the hold time can be significant. Isothermal grain growth of UO₂at 1500° C. is shown in FIG. 9 for various hold times. The average grainsize increased with hold time for a given temperature, where a hold timeof five minutes resulted in a grain size increase of 13% over a pelletthat has a hold time of one minute. Increasing the hold time to tenminutes results in a 53% increase in grain size over the pellet with ahold time of five minutes as is plotted in FIG. 10. As can be seen inFIG. 9, a considerable reduction in pores and pore-densities accompaniesthe increase in grain size, even though the increase in density over a10 minutes period is not significant.

FIG. 11 shows Vickers hardness versus the inverse average grain size.The hardness values increased with decrease in the grain size by theHall-Petch relation. The average hardness value was around 6.4±0.4 GPa.The Young's modulus E and Poisson's ratio v were determined usingultrasonic measurement. The correlation between the longitudinalvelocity (V_(L)), shear velocity (V_(S)), and density (ρ) are given byEquations 1 and 2, below:

$\begin{matrix}{v = \frac{1 - {2\left( {V_{s}/V_{L}} \right)^{2}}}{2 - {2\left( {V_{s}/V_{L}} \right)^{2}}}} & (1) \\{E = {\frac{V_{L}^{2}{\rho\left( {1 + \upsilon} \right)}\left( {1 - {2\upsilon}} \right)}{1 - \upsilon}.}} & (2)\end{matrix}$The Young's modulus was calculated using equations (1) and (2) and thevalues are plotted in FIG. 12, which revealed a linear relationshipbetween the Young's modulus and the relative density of the pellets. Theaverage Young's modulus for pellets of density above 95% is around204±18 GPa.Additional UO₂ Pellet Studies

Uranium dioxide powder was supplied by Areva Fuel System, Hanford, Wash.The powder was reported to have a bulk density of 2.3 g/cm³, tap densityof 2.65 g/cm³, mean particle diameter of 2.4 μm, and a BET surface areaof 3.11 m²/g. The grain size was determined using high resolution SEM tobe around 100-400 nm. The O/U ratio for the starting powder wasdetermined to be UO_(2.16) by measuring the weight change before andafter reducing the powder to stoichiometric UO₂ using ASTM equilibrationmethod (C1430-07).

Sintering was performed using a Dr. Sinter® SPS-1030 system. Thestarting powder was loaded into a 12.5 mm inner diameter graphite dieand placed in the sintering chamber of the SPS which was depressurizedto 10 Pa. A pyrometer was used for monitoring the actual temperature ofthe die surface during the sintering process. Two heating rates of 50and 200° C./minutes were used and a uniaxial pressure of 40 MPa wasapplied when the maximum sintering temperature was reached and held fordesired duration of time. The maximum sintering temperature was variedfrom 750 to 1450° C. and the hold time was varied from 0.5 minutes to 20minutes to achieve different grain sizes in the microstructures.

After sintering, most the sintered pellets were reduced into UO_(2.00)in a furnace at 800° C. for 6 hours, in a 4% H₂—N₂ gas, with a watervapor atmosphere using a water bath maintained at 35° C. The O/U ratioof the resulting pellets was estimated by measuring the weight changebefore and after the reduction process. The density of the reducedpellets was measured using the Archimedes method by immersing thepellets into the distilled water. X-ray Diffraction (XRD, Philips APD3720) was conducted to detect the possible formation of intermetallicsafter sintering. A field emission scanning electron microscopy wasconducted to image microstructural features. Grain size was measuredfrom several micrographs using the line intercept method and observationof the fracture surface in SEM.

Thermal conductivity measurement was calculated using the relationshipk=C_(p) ρ a where k is the thermal conductivity (W/m K), C_(p) is theconstant-pressure specific heat (J/kg·K). ρ (g/cm³) is density and a(cm²/s) is thermal diffusivity. The thermal diffusivity was measured atthree temperatures, 100° C., 500° C. and 900° C. under N₂ atmosphereusing laser flash method (Anter Flashline 3000). Before measurement, thesintered pellets were sectioned into disks of thickness of 3 mm. Bothsurfaces of the disks were coated with the colloidal graphite spray toensure constant heat absorption during the measurement. The laser flashmethod utilizes xenon pulse shot to generate heat on the front surfaceof the disc specimen and the temperature rise on the rear surface isrecorded. The thermal diffusivity (a) is calculated by measuring thespecimen thickness (L) and the time (t_(0.5)) for the temperature of therear face of the disk to rise to the half of its maximum value

$\left( {\alpha = \frac{1.38L^{2}}{\pi\; t_{0.5}}} \right).$Due to the difficulty to directly measure the specific heat, thetheoretical specific heat for UO₂ is used for the calculation, which is258 (J/kg·K), 305 (J/kg·K) and 314 (J/kg·K) for 100° C., 500° C. and900° C., respectively.Density and Grain Size

The influence of hold time at different maximum sintering temperatureson the density of the sintered pellets is plotted in FIG. 13. Below 95%TD, densification can be enhanced either by increasing the maximumsintering temperature or hold time. At 750° C., the pellet density isonly 76% TD when the hold time is 0.5 min. By increasing the hold timeto 20 min, the TD increased to 95%. In addition, the density can also beincreased to 96% by increasing temperature to 1050° C. with only 0.5minute hold time. Thus, one can increase the density of pellet either byincreasing the hold time at a lower sintering temperature or byincreasing the temperature but for shorter hold time. However, afterreaching 95% TD, increasing neither the maximum sintering temperaturenor hold time may result in further significant densification. As aresult, all the densities achieved in this study are below 98% TD.

The influence of hold time and maximum sintering temperature on grainsize is plotted in FIG. 14. At 750° C. with a hold time of 0.5 minute,the resulting average grain size is only 0.2 μm which is the same asthat of the starting powder (0.1-0.4 μm). Even after increasing the holdtime to 20 minutes at this temperature, the average grain size isincreased to only 0.9 μm. On the other hand, by increasing the maximumsintering temperature to 1050° C. for 0.5 minute hold time, the grainsize increases to 3 μm and with further increase in maximum sinteringtemperature to 1450° C., the grain size increased significantly to 6.3μm with a hold time of only 0.5 minute. At a slightly lower temperatureof 1350° C., the maximum grain size of 7 μm was achieved with a holdtime of 20 minutes. At a given maximum sintering temperature, increasein hold time has marginal effect on grain size. But for a given holdtime, increase in sintering temperature has dramatic effect. Furtherincrease in maximum sintering temperature beyond 1450° C. causes largecracks and eventual crumbling of pellets. The details of processingconditions and the resulting density and grain size of the pellets areprovided in Table 1, below.

The correlation between the grain size and the density of the sinteredUO₂ pellets for all the sintering runs with different hold times andmaximum sintering temperatures is plotted in FIG. 15. The grain size ofUO₂ appears to be a function of the pellet density regardless of thehold time and maximum sintering temperature. The curve implies thatduring the early stage of densification until around 90%, there isalmost no grain growth. The average grain size remains below 0.6 μmuntil the theoretical density reaches 90% and it increases to 0.9 μm at95% TD. Beyond this TD, the grain size increases dramatically to 3 μmwhile there is only a slight increase in the TD. The grain size reachesto almost 7 μm when the density reaches close to 97% TD.

TABLE 1 SPS processing parameters and the resulting properties of UO₂pellets Max. Heating rate Hold time Grain size Temp. (C) (° C./min)(min) TD (%) (μm) 750 200 0.5 77.6 +/− 2.0 0.2 +/− 0.1 5 84.5 +/− 0.30.4 +/− 0.1 10 90.0 +/− 0.3 0.6 +/− 0.1 20 95.1 +/− 0.4 0.9 +/− 0.1 8500.5 86.1 +/− 0.9 n/a 1 94.7 +/− 0.6 n/a 5 96.4 +/− 0.1 n/a 20 95.7 +/−1.4  2 +/− 0.5 1050 0.5 96.3 +/− 1.1 3.0 +/− 0.7 5 96.3 +/− 0.9 3.4 +/−0.4 20 95.9 +/− 0.5 4.2 +/− 0.5 1150 0.5 95.4 +/− 0.9 3.9 +/− 0.8 5 95.6+/− 0.5 4.7 +/− 0.9 1350 0.5 96.5 +/− 0.1 5.0 +/− 0.7 5 96.0 +/− 0.1 5.6+/− 1.4 20 96.6 +/− 0.5 6.9 +/− 1.8 1450 0.5 95.8 +/− 1.0 6.3 +/− 1.4850 50 20 96.9 +/− 0.2 2.5 +/− 0.8 1350 96.9 +/− 1.0 8.9 +/− 1.4O/U Ratio

Although the starting O/U ratio of the UO₂ powder was 2.16, depending onthe process conditions, the O/U ratio in the sintered pellet varied. Asseen in FIG. 16, the O/U ratio decreased moderately with increasing thehold time but more severely with the maximum sintering temperature. Atlow sintering temperature of 750° C., only moderate decrease of O/Uratio is revealed when extending the hold time from 0.5 to 20 minutes.However, with the increase in the maximum sintering temperature to 850°C., the O/U ratio dropped more rapidly, and at 1450° C., only 0.5 minutehold time was needed for O/U ratio to reach the desired 2.00. No furtherdecrease in O/U ratio was observed in the range of processingconditions. Chemical reaction occurs during sintering to reduce theoxygen level in the powder.

Thermal Diffusivity and Conductivity

Thermal diffusivity data for the sintered samples are plotted in FIG. 17at three different temperatures. For all the pellets, thermaldiffusivity decreases when the operating temperature increases. However,significant difference is seen among the samples with differentprocessing conditions at each operating temperature. The pelletsprepared at 750° C. with a hold times of 0.5 and 5 minutes have thelowest values of only 0.02 cm²/s at 100° C. while the one sintered at1350° C. for 20 minutes hold time showed a diffusivity of 0.033 cm²/s,an increase of 65%. However, with the increase in temperature, thisdifference tended to decrease. At 900° C., the lowest diffusivity is0.008 cm²/s and the highest value is 0.011 cm²/s, an increase of only38%. Using the diffusivity values presented in FIG. 17, we calculatedthermal conductivity and plotted the value in FIG. 18 as a function oftemperature. The thermal conductivity versus temperature reveals a trendsimilar to that of diffusivity in FIG. 17. The specific heat used in thecalculation was obtained from J. K. Fink, Journal of Nuclear Materials,279 (2000) 1-18. As seen in FIG. 17 and FIG. 18, higher sinteringtemperature and longer hold time are advantageous to produce highthermal diffusivity and conductivity.

Microstructure Development

The final microstructure in the sintered pellet is a function of thelevel of densification and grain growth behavior during the sinteringprocess. As shown in FIG. 15, the grain size is only a function of thedensity regardless of the hold time and maximum sintering temperature.The delay in the grain growth until 95% TD is reached, as shown in FIG.15, may be due to the pinning effect of the inter-granular pores presentin the grain boundaries as revealed in FIG. 19 (Aa, Ab, and Ac), wherethe micrographs in the left column show the microstructure of constantgrain size of about 0.4 μm but the TD values from 77% to 90%. Largeportions of the porosity are clearly seen at these low densities in FIG.19 (Aa-Ac). The pores surrounding each grain inhibit the grain boundarymigration, thus limiting the densification process. With increasingtemperature or hold time, the densification continues where most poresshrink and close due to the grain boundary diffusion. Some pores remainattached to the grain boundary with grain boundary migration. Only asmall portion of the pores are left within the grain, formingintra-granular pores as shown in FIG. 19 (Ba-Bc). As the grain sizecontinues to increase, the density remains almost constant at 96-97% TD.The residual porosity now mainly consisted of intra-granular porosity asseen in the high-magnification images on the right column of FIG. 19.The elimination of intra-granular porosity is possible by latticediffusion which requires longer processing time. Thus, the presence ofintra-granular porosity limits the final density of UO₂ to 97% TD underthe current processing conditions. The formation of intra-granularporosity during densification can be clearly observed in FIG. 20 wherethe high magnification SEM image of a pellet sintered at 750° C. and thedensity is 77% is shown. As indicated by the arrows in the image,various sequences of mechanisms which are operative during grain growthand subsequent densification are shown in this figure. Initially, neckformation occurs between two grains. With the surrounding grains formingsimilar necks simultaneously, grain growth occurs and inter-granularpores were formed. As the densification and grain growth-induced grainboundary migration continues, some pores shrank and closed while someothers separated from the grain boundaries and were left inside thegrain, forming intra-granular pores as revealed in the figure. Therelationship between grain size and densification, as shown in FIG. 15results because during the early stage, the starting powder particlescannot grow into large grains as they are farther apart but withincrease in pressure and temperature, the grains are close and start tomerge due to neck formation. This process eventually reduces porosityand allows densification to occur with simultaneous grain growth, asillustrated in FIG. 15. The ability to fabricate pellets with controlledintra-granular porosity is of significant value when these pellets arein a reactor. Fission gas release can be dropped effectively in theseintra-granular pores, whereas inter-granular pores permit the fissiongas trapped in these pores to cause pellet cracking. Thus, SPS offersthe benefit of control of porosity in the UO₂ pellet.

Reduction of O/U Ratio During SPS

Stoichiometry of UO₂ plays a critical role in pellet physical andthermo-mechanical properties such as grain size, creep resistance andthermal conductivity. A slight deviation in the oxygen/uranium (O/U)ratio from 2.00 can result in significant decrease in thermalconductivity. For efficient operation of UO₂, the O/U ratio of 2.00 mustbe maintained in pellet after the fabrication in reactor environment. Tomaintain this optimal O/U ratio, each sintered pellet can be reduced ina H₂ atmosphere. In a conventional oxidative sintering (which takesalmost 24 hours), the pellet O/U ratio after sintering is around 2.25.This is because this hyperstoichiometry may enhance sinterability ofUO₂. However, the oxidative sintering results in hyperstoichiometric UO₂pellet which requires subsequent post-sintering reduction in H₂atmosphere as per ASTM(C1430-07). FIG. 16 shows that after SPS, thepellets revealed an O/U ratio lower than that of the starting powderUO_(2.16). Additionally, it is noted that, at a maximum sinteringtemperature of 1350° C. and a hold time of 5 minutes, the O/U ratio of2.00 is achieved, with no need for reduction. By establishingappropriate processing conditions, UO₂ powder can be reduced into thestoichiometric form of UO_(2.00) without any need for post-sinteringreduction step in H₂ atmosphere. The reduction of UO₂ may occur from thechemical reaction between graphite punch/die and the powder. As shown inFIG. 21, the XRD curve of the as-sintered pellet at 1450° C. and 0.5minute hold time revealed the formation of uranium carbide on thesurface. However, after hand-grinding the surface layer on each pelletwith 400 grit SiC paper for 1-2 minutes, only UO₂ peaks were detected.Hence, a layer of reaction product between punch/die and UO₂ powder isformed on the surface of the pellet. Based on the U—C—O phase diagram at1000° C., the following reactions are possible in the generatedintermetallic surface layer.UO_(2+x) +xC→UO₂ +xCO  (3)UO₂+4C→UC₂+2CO  (4)UC₂+UO₂→4UC+2CO  (5)Therefore by removing the surface reaction layer, a pellet with adesired UO_(2.00) is achieved.Influence of Density and Grain Size on Thermal Conductivity

As shown in FIG. 15, there are two distinct mechanisms that areoperative during the sintering process: densification dominated below90% TD and grain growth dominated between 96˜97% TD. Thus, the pelletswith the same grain size in the densification phase are chosen to studythe influence of density on thermal conductivity. As seen in FIG. 22,the thermal conductivity is increased with increase in density at allthree operating temperatures. The heat transport in UO₂ is generallythrough the lattice phonon-phonon scattering at the low temperature(<1700° C.). The existence of porosity in the structure and thegas-solid interfaces, as well as the poor thermal conductivity of thegas inside the pores, prevents the heat transfer efficiency inlow-density UO₂. Temperature is another important factor influencing thethermal conductivity in UO₂. With the increase in temperature, the meanfree path of phonon is decreased and phonon-phonon scattering is furtherdisturbed, leading to further reduction of resulting thermalconductivity.

The correlation between grain size and thermal conductivity for all thepellets is plotted in FIG. 23. In order to get a more comprehensiveunderstanding of the influence of grain size, additional SPS runs withslower heating rate (50° C./min) have been conducted and the processingconditions for these pellets are also listed in Table 1. As seen in FIG.23, by varying the grain size from 2 to 9 μm, no significant differencein thermal conductivity was noted at all the three temperatures.Although the smallest grain size in this study is only 2 m, comparedwith 10-15 m in conventional sintering methods, there is only slightdecrease in thermal conductivity at 100° C. when the grain size is lessthan 4.5 μm and no influence of grain size is revealed at 500° C. and900° C. The dependence of thermal conductivity on grain size stems fromthe interfacial resistance, which is measured in terms of Kapitzalength. When the grain size is smaller than or comparable to Kapitzalength which is ˜100 nm at 100° C. for UO₂, the grain itself offers thesame thermal resistance as the interface. Thus, the thermal conductivityis mainly dominated by grain sizes in this regime. Further decrease ofthe grain size may strongly decrease the thermal conductivity. On theother hand, when the grain size is larger than the magnitude of Kapitzalength, the effect of the grain size is only due to the interfacialresistance at the grain boundaries. With larger grain size, there isless volume of grain boundaries that can act as the barrier to preventthe heat transport. Thus, higher thermal conductivity is expected forlarger grain size. In our case, the typical grain size is more than 10times larger than Kapitza length, which indicates that the increase inthermal conductivity is mainly due to the decreased volume of grainboundaries, however, only marginal increase of thermal conductivity ofUO₂ when the grain size is larger than 1 μm at 300K. This conclusion isin agreement with the result presented in our study. The average valuesof thermal conductivity for these pellets, 8.2 (W/mK), 4.7 (W/mK) and3.4 (W/mK), at three operating temperatures are indicated by dashedlines in FIG. 23, which is near the maximum values reported in theliterature for conventionally sintered UO₂ pellets, indicated by theshaded areas in FIG. 23. The average thermal conductivities for sampleswith average grain sizes above 4.5 μm are higher than the maximum ofthat reported for conventional sintering.

The advantages of SPS over conventional sintering is summarized in Table2, below. These features are expected to yield significant economicbenefit if large scale manufacturing using SPS can be implemented.

TABLE 2 Comparison of SPS and conventional sintering methods StageFeature SPS Conventional Pre- Modify starting Not required Requiredsintering powder Cold compaction of Not required Required green bodySintering Temperature ramp 100-200° C./min 1-5° C./min rate Maximumsintering 1050° C. for UO₂ 1600° C. temperature Hold time 0.5-5 min 4hrs Total sintering run <1 hr ~15 hrs. time Pressure 20-80 MPa NoSintering Vacuum (~10 Pa) Gaseous environment Dimensional control YesLimited Stoichiometry Changed Unchanged during sintering Control ofGrain High Low growth Produce near net Yes yes shape pellets Usedifficult-to- Yes limited sinter materials Post- Reduction of Powderstoichiometry Yes sintering sintered pellet dependent Additional Notnecessarily Yes machining neededUO₂—SiC Composite Fuel Pellets

Uranium dioxide powder was supplied by Los Alamos National Laboratory(Los Alamos, N. Mex.). All sintering was performed using the as-receivedUO_(2.11) powders. β-SiC whiskers (3C—SiC) were obtained from AdvancedComposite Materials, Greer, S.C. (SC-9D, deagglomerated SiC whiskers)and possess an aspect ratio, diameter, and length that exceeds 10:1,0.65 μm, and 10 μm, respectively. β-SiC powder (3C—SiC) having a meandiameter of 1 μm was obtained from Alfa Aesar Inc, Ward Hill, Mass.

In separate runs, either SiC whiskers (SiCw) or SiC powders (SiCp) wereused to produce UO₂—SiC composite fuel pellets. FIG. 24 shows SEM imagesof as received $-SiC whisker and powder morphologies. UO₂ and 10 vol %(about 3.24 wt %) SiC were blended for 1 hour with the aid of2,3-dihydroperfluoropentane using a SPEX 8000 shaker. After mixing, theblending aid was allowed to evaporate in a fume hood, leaving noresidual contamination. This process resulted in homogeneous dispersionof SiC whiskers and powder particles in UO₂ matrix.

Spark plasma sintering (SPS) and traditional oxidative sinteringprocesses were employed for comparison purposes. Green body pellets weremade for oxidative sintering by compressing the blended UO₂—SiC powderat 200 MPa for 10 minutes in a stainless steel die. The die walls werelubricated with a film of stearic acid to prevent fracture of 12.7 mmgreen body pellets while removing from the die. The green body pelletswere sintered in an alumina tube furnace with a ramp rate of 2.6°C./minutes until the temperature reached 1600° C. where the temperaturewas maintained for 4 hours. To maintain a hyper-stoichiometric state, anultra high purity (UHP) Ar gas atmosphere was established by acontinuous flow of Ar at a rate of 2 liter/minutes in the tube furnaceduring sintering.

Spark plasma sintering was performed in a Dr. Sinter® SPS-1030 systemhaving the die assembly shown in FIG. 1. For SPS, the UO₂—SiC blendedpowder was loaded into the 12.7 mm diameter graphite die, with the innerdie surface covered by a thin graphite foil to prevent reaction of theUO₂ with the die wall. Cylindrical graphite plugs were inserted intoboth ends of the die. The end of each plug that contacts the blendedpowder was coated with an aerosol of graphite (ZYP Coatings, Inc., OakRidge, Tenn.) to prevent reaction. The ramp-up/down rate was set at 100°C./minutes and the hold time at maximum temperature was set at 5minutes. An axial pressure of 40 MPa was applied at the beginning of thehold time. Maximum sintering temperatures were set at 1400, 1500, and1600° C. for different fuel pellets.

After removal of the fuel pellets from the sintering chamber, reductionto stoichiometric UO₂ was carried out by the procedure in ASTM C1430-07. Thermal treatment for the reduction was conducted in a furnaceat 800° C. for six hours, in a 4% H₂—N₂ gas with water vapor added bypassing the gas through a 35° C. water bath. For comparison purposes UO₂pellets were prepared in parallel using identical SPS and oxidativesintering conditions.

The density of the UO₂—SiC composite pellets was measured on paraffinwax coated pellets using the Archimedean immersion method. The paraffincoated pellet was weighed three times in water and the average densitywas calculated.

Fuel pellets were polished with successively smaller grinding mediumwith a minimum 0.04 micron colloidal silica used for the final polish.Grain boundary relief was produced by thermal etching at 1340° C. for 4hours in an Argon atmosphere. SEM (JEOL 6335F), micrographs of theUO₂—SiC fuel pellets were taken using the secondary electron mode withthe average grain size determined by the line intercept method. Todetermine elemental diffusion ranges, penetration curves of U and Sialong a line normal to the interface of UO₂—SiC were obtained by EnergyDispersive X-ray Spectroscopy (EDS) coupled with high resolution FE-SEM.

Reaction products formed upon sintering were determined by X-RayDiffraction (Philips APD 3720) on the pellets for the composite fuelpellets having UO₂-70 vol % (41.27 wt %) SiC pellets prepared in themanner of the UO₂-10 vol % SiC pellets.

Thermal conductivities of the fuel pellets were measured using an AnterFlashline®3000 system, where the derivation of thermal diffusivity, a,and specific heat capacity, C_(p), were based on the measurement of therising temperature on the back surface of a sample caused by a pulsedlaser beam on the sample's front surface. Measurements were performed intriplicate at 100, 500, and 900° C. from which the average conductivityat each temperature was calculated. Thermal diffusivity, α, in m2/s, isgiven by, 0.1388L2/t_(1/2), where L is the thickness of the specimen inm, and t_(1/2) is the time in seconds for the rear surface temperatureto reach 50% of its maximum value. The specific heat capacity, C_(p), isgiven by Q/dT·m, where Q is the energy of the pulsed laser beam,determined by comparing the maximum value of the temperature rise tothat of a reference, m is the mass of the specimen, and dT is themaximum value of the temperature rise. Pyroceram, a certified referenceglass-ceramic material, was used as the reference pellet due to itssimilar conductivity with UO₂. By multiplying density with α and Cp,thermal conductivity was calculated.

UO₂ pellets produced via oxidative sintering (a) and SPS (b) are shownin FIGS. 25A and 25B, respectively. Each pellet was of 12.5 mm diameterand 2-4 mm thick. Pellets were cut and prepared for characterization asindicated above. The measured relative densities of the oxidative andSPS sintered UO₂—SiC fuel pellets prepared with various maximumsintering temperatures are shown in FIG. 26. The density of sinteredUO₂-10 vol % SiC pellets increased with the maximum sinteringtemperature. The highest density for an oxidative sintered pellet was88.91% TD. In contrast, the SPS sintered fuel pellets having a maximumsintering temperature of at least 1400° C. displayed densities between91.25 and 97.78% TD.

FIG. 27 shows the distributions of SiCw and SiCp in the composite fuelpellets. Both the whiskers and particles appear as uniformly distributedwithout agglomeration. This is attributed to the use of2,3-dihydroperfluoropentane as a dispersing agent during green compactpreparation. UO₂-10 vol % SiC fuel pellets sintered at 1500° C. usingSiCw and SiCp and oxidative sintering and SPS develop themicro-morphologies shown in FIG. 28. Greater porosity and poorinterfacial contact were observed for fuel pellets prepared by oxidativesintering than for the pellets prepared using SPS. Good interfacialcontact is very beneficial if a highly thermal conductive fuel pellet isdesired, as the presence of voids at the interface of two grains or poorinterfacial contact between the two phases inhibits heat transfer. Thehigh level of porosity in the oxidative sintered pellet, shown in FIG.28 a) and c) is also reflected in the measured density of the fuelpellets, as illustrated in FIG. 26. The density and interfacial contactis greater for the fuel pellet compositions sintered by SPS, asindicated in FIG. 7b ) and d). The improved interfacial contactillustrates the advantage of SPS for sintering for high thermalconductivity UO₂—SiC pellets, according to an embodiment of theinvention.

FIG. 29 shows a high magnification of a UO₂—SiC interface for a SPSsintered fuel pellet prepared with a maximum temperature of 1600° C.maintained for 5 minutes. The separation between the two phases isnormally less than 100 nm wide. EDS line scanning was performed todetermine the uranium and silicon concentration profiles across theinterface. The concentration profile, shown in FIG. 29, displays anapproximately 3 μm interpenetration of the two elements. These profilesalso illustrate that uranium penetrates to about 1.17 μm into the SiCphase, whereas Si penetrates into the UO₂ by about 1.83 μm. Thus uraniumpenetrates SiC approximately 36% less than does SiC into UO₂. If oneassumes that both materials have similar proportions of vacancy defectsin their lattice, this penetration difference is consistent with thegreater atomic density and weight of uranium relative to that ofsilicon.

Controlling reactions between SiC and UO₂ during high temperaturesintering is critical for achieving useful UO₂—SiC pellets, because theformation of reaction products at the UO₂—SiC interface may lead to poorthermal properties. XRD was employed to analyze for reaction products atthe UO₂—SiC interface. FIG. 30 shows XRD spectra obtained from oxidativesintered UO₂-70 vol % SiC pellets held at 1600° C. for four hours andSPS 1600° C. for five minutes. A USi_(1.88) peak is clearly seen in theoxidative sintered fuel pellet but is not detected for the fuel pelletfabricated by SPS. In the SPS, the pellet stays above 1370° C. for only9.6 minutes as opposed to 6.9 hour hold for oxidative sintering. Longsintering times allow the diffusion of chemical species, formation ofintermetallics, and gas phases, such as CO or CO₂. Intermetallicsincrease the number of phonon scattering cites at the interfaces. Gasphases may hinder the interfacial contact of UO₂—SiC by forming voids orcausing separation. Both of these defects can significantly reduce thethermal conductivity of the composite pellet.

The average grain size in fuel pellets sintered at 1500° C. by bothoxidative sintering and SPS is shown in the bar graph of FIG. 31. Ineach pellet, the average grain size was determined from threemicrographs from different regions of the pellets. UO₂ fuel pellets freeof SiC display the greatest grain size. The grain size decreases withsilicon carbide additions using either sintering method. This isconsistent with insoluble second-phase particles dispersed randomly in apolycrystalline solid that pin the grain boundary movement. UO₂-SiCppellets processed via oxidative and SPS sintering display 62% and 68.5%smaller grains, respectively, than the grains displayed by the SiC freeUO₂ fuel pellet. In general, SPS fuel pellets have smaller grain sizesthan oxidative sintered pellets, which have much greater time frames forgrain growth. A 53.3% smaller grain size was observed for the UO₂ pelletmade by SPS than in oxidative sintered UO₂ pellet. UO₂-SiCp fuel pelletsdisplay a smaller grain size than do UO₂-SiCw fuel pellets. The meanvolume of a single SiCw whisker is 3.32 μm³, (0.65/2)²×π×10, while themean volume of a SiCp powder particle is 0.39 μm³, (0.5)³×π. ThereforeSiCw comprising fuel pellets require only about 12% of the number ofparticles as the SiCp comprising fuel pellets for an equivalent volumeof SiC in the fuel pellet, and the grain size is inversely affected bythe amount of the SiC particles.

Thermal conductivity measurements were carried out on UO₂—SiC fuelpellets sintered by oxidative sintering at 1600° C. and SPS at varioustemperatures. Measurements were carried out in triplicate at 100, 500,and 900° C. for each fuel pellet composition and the averageconductivity values were calculated. The average thermal conductivityvalues of UO₂ from Fink, J. Nucl. Mater. 2000 279, 1-18. and the valuesdetermined by the method indicated above, are plotted in FIG. 32. SPSsintered UO₂—SiC composite fuel pellets revealed higher thermalconductivities than did UO₂ pellets. In general, UO₂—SiC composite fuelpellets with the higher observed thermal conductivities are observed forpellets processed at higher SPS sintering temperatures. Oxidativesintered pellets exhibited significantly lower conductivity values thandid the UO₂ pellets. Maximum thermal conductivity enhancement wasobserved in UO₂—SiC composites sintered by SPS at 1600° C. with 54.9,57.4, and 62.1% greater conductivity at 100, 500, and 900° C.,respectively, compared to the literature UO₂ values. The SPS sinteredcomposite pellets show a trend similar to that of UO₂ with respect totemperature, where a gradual decrease in conductivity was observed withincreasing temperature. There is no significant difference in thethermal conductivity values observed for UO₂-SiCw and UO₂-SiCpcomprising fuel pellets.

Additional Fabrication of UO₂—SiC Composite Pellets

Uranium dioxide (UO_(2.11)) powder was obtained from Areva, Hanford,Wash. and the SiC powder was obtained from Superior Graphite, Inc.,Chicago, Ill. The reported SiC particle mean diameters were 0.6, 1.0,9.0, 16.9, and 55 μm. The UO₂ and SiC powders were mixed in a ceramicvial with stainless steel balls and a blending aid,3-dihydroperfluoropentane, and blended in a SPEX 8000 shaker for onehour. For each mixing run the SiC mean particle size and the volumefraction of SiC powder in the mixture with UO₂ were varied as shown inTable 3, below to investigate their effect on the thermal conductivityof the resulting UO₂—SiC composite pellet. The SiC particles are highpurity (>98%) powders. SiC particles with 1 μm size at 5, 10, 15, and 20vol % were chosen to fabricate UO₂—SiC composite pellets. SiC dispersedin UO₂ powders were then sintered using a Dr. Sinter® SPS-1030 system at1350° C. and 1450° C. for 5 minutes in a vacuum (˜30 mTorr). The rampup/down rate and mechanical pressure at the maximum sinteringtemperature were held constant at 100° C./min and 36 MPa, respectively.Treatment according to ASTM C 1430-07 was conducted on the sinteredcomposite pellet to reduce UO_(2+x) to stoichiometric UO_(2.00). Theramp up/down rate and maximum temperature were set at 2.6° C./min and800° C., respectively. The heat treatment was performed in a Lindberg®alumina tube furnace using 4% H₂—N₂ gas with the water vapor atmospheremaintained at 35° C.

TABLE 3 Details of SiC particle size, volume fraction, and sinteringconditions in the SPS.* Maximum SiC particle SiC volume sintering % TDof the mean diamter fraction temperature composite (μm) (%) (° C.)pellet ± SD 0.6 5 1350 95.25 ± 0.24 1 5 1350 95.27 ± 0.3  1 5 1450 96.81± 0.39 1 10 1450 96.63 ± 0.35 1 15 1450 95.14 ± 0.23 1 20 1450 94.41 ±0.3  9 5 1350 95.15 ± 0.09 16.9 5 1350 94.75 ± 0.17 55 5 1350  95.1 ±0.13 *Hold time = 5 mins; ramp up/down rate = 100° C./min; pressure = 36MPa.

The weight of each pellet in air and water was measured and the averagedensity was calculated from three weight measurements per pellet usingthe Archimedes principle. The measured density of the composite was thencompared with theoretical density obtained from the rule of mixture.ρ_(c)=ρ_(UO) ₂ (1−V _(p))+ρ_(SIC) V _(p)  (6)where ρ_(UO2), ρ_(SiC), and V_(p) are the densities of UO₂ and SiC, andthe SiC volume fraction, respectively.

The microstructure of the fabricated composite pellets was observedusing a scanning electron microscope (SEM, JEOL JSM-6335F). The pelletswere metallographically polished with successively smaller grit SiCabrasive paper and finally with 0.06 μm colloidal silica. The surfacewas thermally etched at 1340° C. in Ar atmosphere for 4 hours to revealthe grain boundaries of UO₂ matrix in the composite pellets. Themeasurement of thermal diffusivity was carried out at 100, 500, and 900°C. using a laser flash instrument (AnterFlashline®3000) with a Xenondischarge pulse for 1 μs duration. Three measurements were performed ateach temperature on each pellet and the average diffusivity wasobtained. The specific heat capacity of UO₂—SiC composite pellet wascalculated using the Neumann-kopp rule:C _(p) =C _(UO) ₂ (1−f _(p))+C _(SiC) f _(p)  (7)where C_(UO2), C_(SiC), and f_(p) are theoretical specific heatcapacities of UO₂ and SiC, and weight fraction of SiC particles,respectively, at a specific temperature. C_(UO2) and C_(SiC) at 100° C.,500° C., and 900° C. are listed in Table 4. The thermal conductivity, K,of composite pellets was then determined from the relation:K=DC _(p)ρ_(c)  (8)where D and ρ_(c) are the thermal diffusivity and density of thecomposite, respectively.

TABLE 4 Thermal properties of UO₂ and SiC Thermal expan- sion coeffi-Specific heat, Thermal cient C_(p) (J/kg · K) Conductivity, K Mate-(K⁻¹) at at at rial 25° C. 100° C. 500° C. 900° C. 100° C. 500° C. 900°C. UO₂ 9.93 × 258.17  304.62  314.17  6.83  4.28  3.01 10⁻⁶ β-SiC  4.4 ×798.2 1139.1 1243.31 273.64 136.42 85.53 10⁻⁶Size Effect of SiC Particles on UO₂-5 Vol % SiC Composite Properties

The micro-morphologies and thermal properties of UO₂-5 vol % SiCcomposite fuel pellets containing SiC particles with five differentsizes (See Table 3) were examined. FIG. 33 shows the microstructures ofthese composites where the SiC particles appear black and the brighterarea indicates the UO₂ matrix. The SiC particles appear to behomogeneously dispersed in the UO₂ matrix in all the composites.However, as shown in FIG. 33(e), in the composite containing 55 μm SiCparticles, distinct radial micro-cracks were observed originating at anedge of the interface between a SiC particle and UO₂ matrix andpropagating towards another SiC particle.

The interfaces between the UO₂ matrix and SiC particles in UO₂-5 vol %SiC composite pellets with different sized SiC grains are shown in FIG.34. The micro-cracks emanating from the SiC particles are clearly seenin FIGS. 34 (c), (d), and (e) indicating that micro-cracks evolve incomposites with SiC particles larger than 9 μm in size. However, themicro cracking is less severe in composites with SiC particles of size 9μm and 16.9 μm compared to the composite containing 55 μm diameter SiCparticles. No visible cracks in the micro structure were seen in thecomposite pellets with smaller size SiC particles.

With increasing particle size there is a larger separation between theSiC particle and the UO₂ matrix. FIG. 35 shows the interfacial debondingbetween UO₂ and SiC particles in each composite with the three largestsize SiC particles. While the interfacial contact between UO₂ grains and9 μm SiC particle is fairly good, a visible gap is observed at theinterface between UO₂ grains and SiC particle when the particle size is16.9 μm or greater.

Micro-Cracking and Interfacial Debonding Occur in Various CompositesDuring the Sintering Process Due to a Mismatch in Coefficients ofThermal Expansion (CTE) Between the Matrix and the Second PhaseParticles.

As indicated in Table 4, the thermal expansion coefficient of UO₂ ismore than twice that of SiC so that the matrix expands into theparticles during the cooling process forcing the SiC particles intocompression. The larger the particle size, the more will be the inducedcompressive stress due to the lower surface area-to-volume ratio of thelarger particles. When the stress intensity at the interface exceeds thegrain boundary toughness of matrix material, spontaneous micro-crackingis initiated from the interface into the matrix in a ceramic composite.The induced internal stress caused by a mismatch in CTE of constituentsin a composite also leads to a partial interfacial debonding. The degreeof interfacial debonding is dependent on the level of mismatch in CTE,elastic properties of the constituents, the temperature range of coolingprocess, and the energy required to create a new surface. Because thesethermal cracks and the interfacial debonding in composite pelletsobstruct the pathway for heat conduction, extensive cracking and poorinterfacial contact obviously lead to lower thermal conductivity.

The measured densities of UO₂—SiC composite fuel pellets containing 5vol % SiC particles but of different sizes are shown in Table 1. Thedensities of all composite pellets are near 95% TD and appear not to bedependent on the SiC particle size. Because the thermal conductivity isdirectly proportional to the density as seen in Eq. (8), and the densityof the composite pellet is not dependent on the SiC particle size, themeasured pellet thermal conductivity will mostly depend on the size ofSiC particles as will be discussed in the following paragraph.

FIG. 36 shows the temperature dependence of the measured thermaldiffusivity for UO₂-5 vol % SiC composite pellets containing varioussizes of SiC particles. The red line refers to the literature value of95% dense UO₂. In general, UO₂ thermal diffusivity decreases withtemperature due to increased phonon-phonon scattering at highertemperatures. This trend is maintained in the thermal diffusivity ofUO₂-5 vol % SiC composite pellets as well. In general, the larger theSiC particle size the lower the thermal diffusivity. However, thethermal diffusivity of the composite pellets containing 55 μm SiCparticles shows a significantly lower thermal diffusivity than theliterature UO₂ value due to extensive micro-cracks and severeinterfacial debonding as shown in FIG. 34 and FIG. 35.

FIG. 37 shows the thermal conductivity determined by Eq. 8 using themeasured thermal diffusivity (FIG. 36), calculated specific heatcapacity, and the measured density (Table 3) for composites withdifferent SiC size particles at three temperatures. The specific heatcapacities of UO₂ and β-SiC, shown in Table 4, were utilized todetermine that of the UO₂-5 vol % SiC composite. Using Eq. 7, thespecific heat capacities at 100, 500, and 900° C. were calculated to be266.5, 317.5, and 328.5 J/kg·K, respectively. While the compositepellets containing 0.6, 1.0, and 9.0 μm diameter SiC particles showedenhanced thermal conductivity, the composite pellets containing 16.9 and55 μm diameter SiC particles revealed lower thermal conductivity than aUO₂ pellet. The SiC particle size dependence of thermal conductivity atvarious temperatures is shown in FIG. 37. While marginal reduction inthermal conductivity in the pellets containing 16.9 μm SiC particles isnoted, the reduction in pellets containing 55 μm diameter SiC particlesis particularly large; a decrease of 21-28.3% depending on the testingtemperature. These composite pellets with poor thermal conductivity andcontaining large SiC particles are the same pellets which exhibitedmicro-cracks and interfacial debonding next to the large SiC particlesas shown in FIG. 34 and FIG. 35. These observations support thehypothesis that micro-cracking and interfacial debonding are responsiblefor the reduction in thermal conductivity of UO₂—SiC compositecontaining large SiC particles. The 55 μm diameter SiC particlecomposite pellet has the most severe micro-cracks and the largestinterfacial debonding resulting in a greatly reduced thermalconductivity.

Effect of Volume Fraction of SiC Particles

To understand of the influence of volume fraction of SiC particles onthe thermal properties of UO₂—SiC composite pellets, we have chosen oneSiC particle size (1 μm) and varied the volume fraction (see Table 3) at5, 10, 15, and 20%. All the other variables were kept constant duringthe sintering process. Micro-structures of the four composite pelletsrevealing homogeneously dispersed 1 μm SiC particles are shown in FIG.38. With increase in volume fraction, particle-particle interaction isnoted as seen in FIGS. 38 (c) and (d).

FIG. 39 reveals a decrease in the relative density of UO₂—SiC compositepellets with increasing SiC volume fraction. Increases in particlevolume fraction hinders the consolidation of matrix grains.

Measured thermal diffusivity of the composite pellets containing variousvolume fractions of 1 μm size SiC particles is shown in FIG. 40. Highervolume fraction of SiC particles results in a higher thermal diffusivityof the composite. This trend indicates that inclusion of higher thermalconducting particles into UO₂ matrix increases the diffusion of heatenergy in the composite provided good interfacial bonding is maintained.

FIG. 41 reveals the calculated (Eq. 7) specific heat capacity (Cp) ofUO₂—SiC composites containing various volume fractions of SiC particlesat 100, 500, and 900° C. As the SiC volume fraction and temperatureincrease the specific heat capacity also increases. This is consistentwith a larger number of molecular energy states available at highertemperature and the specific heat capacity follows Neumann-kopp rule(Eq. 7). Initially there is a significant increase in specific heat from100 to 500° C. but this increase in Cp is lower from 500 to 900° C.

Hasselman et al. Journal of Composite Materials 1987; 21:508-15 providean expression for calculating the effective thermal conductivity of acomposite. For a composite containing spherical shaped particlesdispersed homogeneously in a matrix material, the effective thermalconductivity is given by:

$\begin{matrix}{k_{eff} = {k_{m}\frac{{2\left( {\frac{k_{p}}{k_{m}} - \frac{k_{p}}{{ah}_{c}} - 1} \right)V_{p}} + \frac{k_{p}}{k_{m}} + {2\frac{k_{p}}{{ah}_{c}}} + 2}{{\left( {1 - \frac{k_{p}}{k_{m}} + \frac{k_{p}}{{ah}_{c}}} \right)V_{p}} + \frac{k_{p}}{k_{m}} + {2\frac{k_{p}}{{ah}_{c}}} + 2}}} & (9)\end{matrix}$where k_(eff) is the effective thermal conductivity, subscripts p and mare particle and matrix, respectively, V_(p) is the volume fraction ofparticles, a is the radius of particle, and h_(c) is the interfacialthermal conductance. The reported interfacial thermal conductance h_(c)accounting for the UO₂—SiC interface has not been reported to ourknowledge. However, it can be estimated using the acoustic mismatchmodel of Swartz et al. Reviews of Modern Physics 1989; 61:605-68, wherethe interfacial thermal conductance is:

$\begin{matrix}{h_{c} \approx {\frac{1}{2}{\rho_{m} \cdot C_{p} \cdot \frac{v_{m}^{3}}{v_{p}^{2}} \cdot \frac{\rho_{m}\rho_{p}v_{m}v_{p}}{{\rho_{m}v_{m}} + {\rho_{p}v_{p}}}}}} & (10)\end{matrix}$where ρ is the density C_(p) is the specific heat capacity of matrix, vis the phonon velocity, and subscript p and m refer to particle andmatrix, respectively. The phonon velocities of UO₂ matrix and SiCparticle can be estimated from the equation:

$\begin{matrix}{{\frac{1}{v_{l}^{2}} + \frac{2}{v_{t}^{2}}} = \frac{3}{v}} & (11)\end{matrix}$

Using input parameters with particle volume fraction (0.05-0.2), radius(0.5 μm), as listed in Table 1 into Hasselman and Johnson model (Eq.(9)), the comparison between experimentally obtained and theoreticallycalculated effective thermal conductivity is shown in FIG. 42.Experimentally measured density (FIG. 39), thermal diffusivity (FIG.40), and the calculated specific heat (FIG. 41) were utilized todetermine the experimental effective thermal conductivity using Eq. (8).The higher the volume fraction of SiC particles, the higher the thermalconductivity of the composite. The average increase in thermalconductivity with the addition of 5, 10, 15, and 20 vol % of SiCparticles are 14.23, 26.44, 43.22, and 49.84%, respectively. Consideringthe error bar, great agreement between experimentally determined andtheoretically calculated effective thermal conductivities is seen forthe composite pellets containing 5, 10, and 15 vol % of SiC particles.The agreement is also much better at higher temperature (900° C.) thanat lower temperature (100° C.).

The lower experimental thermal conductivities of 20 vol % SiC canreflect the relatively lower densification and the abundance ofparticle-particle interactions of the composite containing higher volumefraction of SiC particles. As shown in FIG. 39, the composite containing20 vol % of SiC has only 94.41% relative density and seemed to beresponsible for decreasing the thermal conductivity (see Eq. 8). Theinteraction between SiC particles is seen in FIG. 38 and it is moreabundant with increasing SiC volume fraction. The particle-particleinteraction is not accounted for in Hasselman and Johnson model due tothe complexity of the phenomena. Observation of the interfacial contactbetween SiC particles, shown in FIG. 43, indicates that pores arepredominantly located at the interface reducing the overall thermalconductivity due to the phonon scattering. Non-ideal shape of SiCparticles and thermal diffusivity measurement error also can reflect thedifference between experimental and theoretical effective thermalconductivities. The irregularities in SiC particle shape can be clearlyseen in FIG. 33 and FIG. 34. Because the Hasselman and Johnson model(Eq. 9) only accounts for spherical shaped secondary particles, adiscrepancy between theoretical model and experimental measurement isexpected. Thermal diffusivity measurement error possibly contributes tothe difference between experimental and theoretical effective thermalconductivities. The difference between actual measurement temperatureand the set up temperature and change in the density value of pellet atdifferent temperatures may cause some error to the experimental thermalconductivity.

Relatively good agreement between the experimental and theoreticaleffective thermal conductivities of UO₂—SiC composites are observed. UO₂matrix and 1 μm SiC particles are mechanically well contacted in UO₂—SiCcomposites thus improving the effective thermal conductivity. Moreover,both experiments and the theoretical model revealed that highereffective thermal conductivity is obtained with increasing SiC volumefraction. However, the utilized powder blending procedure and SPSprocess conditions, see Table 3, are only valid for fabrication ofUO₂—SiC composites containing up to 15% SiC particles by volume.

Hence, SPS processed UO₂—SiC, according to an embodiment of theinvention, offers a significantly shorter sintering time to yield adense UO₂—SiC composite fuel pellet having a reduced formation ofchemical products, better interfacial properties, and significantlybetter thermal conductivity than is observed for pellets obtained byoxidative sintering. Although it is generally observed that smallergrain size fuel pellets display lower thermal conductivities, SPSproduced UO₂—SiC fuel pellets had superior thermal conductivities thando oxidative sintered UO₂—SiC fuel pellets that display larger grainsizes. UO₂—SiC composite fuel pellet, according to an embodiment of theinvention, display a high density, good interfacial contact, and noextraneous intermetallics or other chemical by-products produced duringsintering.

All patents, patent applications, provisional applications, andpublications referred to or cited herein are incorporated by referencein their entirety, including all figures and tables, to the extent theyare not inconsistent with the explicit teachings of this specification.

It should be understood that the examples and embodiments describedherein are for illustrative purposes only and that various modificationsor changes in light thereof will be suggested to persons skilled in theart and are to be included within the spirit and purview of thisapplication.

We claim:
 1. A nuclear fuel pellet, comprising a compressed homogeneousdispersion of powders comprising uranium dioxide (UO₂) and a thermallyconductive material selected from a metal or metal alloy, wherein: theUO₂ is 90 to 99 vol % of the compressed homogeneous dispersion ofpowders; the thermally conductive material has a thermal conductivitythat is greater than 10 W/mK at 100° C.; and the compressed homogeneousdispersion of powders has a density in excess of 95% TD (theoreticaldensity) of the UO₂, an average grain size of at least 4 μm with littleor no intergranular pores, and a thermal conductivity of at least 8 W/mKat 100° C.
 2. The nuclear fuel pellet of claim 1, wherein said thermallyconductive material is uniformly distributed with said UO₂.
 3. Thenuclear fuel pellet of claim 1, wherein said thermally conductivematerial comprises 1 to 10 vol % of said compressed homogeneousdispersion of powders.
 4. The nuclear fuel pellet of claim 1, whereinsaid compressed homogeneous dispersion of powders further comprisesother oxidation states of uranium oxide, uranium nitride, thorium oxide,plutonium oxide, or any combination thereof.
 5. The nuclear fuel pelletof claim 1, wherein said compressed homogeneous dispersion of powderscomprises a mixture of the UO₂ with other uranium oxides, the mixturehaving a stoichiometry of UO_(2+x) where x is 0 to 0.25.
 6. The nuclearfuel pellet of claim 1, wherein said compressed homogeneous dispersionof powders consists of UO₂ and said thermally conductive material andsaid thermally conductive material is uranium or a uranium alloy.
 7. Thenuclear fuel pellet of claim 1, wherein the compressed homogeneousdispersion of powders is formed in a die assembly at a controlledpressure of 25 to 100 MPa and temperature of 850 to 1600° C.
 8. Thenuclear fuel pellet of claim 7, wherein the controlled pressure andtemperature are maintained for a period of 0.5 to 20 minutes.
 9. Thenuclear fuel pellet of claim 7, wherein the controlled pressure andtemperature are applied in a sintering chamber of a spark plasmasintering (SPS) apparatus.